Wireless local area networks (WLANS) are gaining in popularity, and new wireless applications are being developed. The original WLAN standards, such as “Bluetooth” and IEEE 802.11, were designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a, 802.11b and 802.11g extensions to the original standard, in order to enable higher data rates. The 802.11a standard, for example, envisions data rates up to 54 Mbps over short distances in a band near 5 GHz, while 802.11b defines data rates up to 22 Mbps in the 2.4 GHz band. In the context of the present patent application and in the claims, the term “802.11” is used to refer collectively to the original IEEE 802.11 standard and all its variants and extensions, unless specifically noted otherwise.
The theoretical capability of new WLAN technologies to offer enormous communication bandwidth to mobile users is severely hampered by the practical limitations of wireless communications. Indoor propagation of radio frequencies is not isotropic, because radio waves are influenced by building layout and furnishings. Therefore, even when wireless access points are carefully positioned throughout a building, some “black holes” generally remain—areas with little or no radio reception. Furthermore, 802.11 wireless links can operate at full speed only under conditions of high signal/noise ratio. Signal strength scales inversely with the distance of the mobile station from its access point, and therefore so does communication speed. A single mobile station with poor reception due to distance or radio propagation problems can slow down WLAN access for all other users in its basic service set (BSS—the group of mobile stations communicating with the same access point).
The natural response to these practical difficulties would be to distribute a greater number of access points within the area to be served. If a receiver receives signals simultaneously from two sources of similar strength on the same frequency channel, however, it is generally unable to decipher either signal. The 802.11 standard provides a mechanism for collision avoidance known as clear channel assessment (CCA), which requires a station to refrain from transmitting when it senses other transmissions on its frequency channel. In practice, this mechanism is of limited utility and can place a heavy burden on different BSSs operating on the same frequency channel.
Therefore, in 802.11 WLANs known in the art, access points in mutual proximity must use different frequency channels. Theoretically, the 802.11b and 802.11g standards define 14 frequency channels in the 2.4 GHz band, but because of bandwidth and regulatory limitations, WLANs operating according to these standards in the United States and most European countries actually have only three non-overlapping frequency channels from which to choose. (In other countries, such as Spain, France and Japan, only one channel is available.) As a result, in complex, indoor environments, it becomes practically impossible to distribute wireless access points closely enough to give strong signals throughout the environment without substantial overlap in the coverage areas of different access points operating on the same frequency channel.
Some WLAN standards provide for transmission power control (also known as “transmit power control,” or TPC). TPC is applied by access points in order to determine the power level of the signals they transmit to mobile stations. It may also be applied by the mobile stations, as well, in transmission to the access points. Typically, in a WLAN, TPC limits the power transmitted by the access point to the minimum needed to reach the farthest mobile station, and it similarly may limit the power transmitted by the mobile station to the minimum needed to reach the access point that is serving it. TPC is mandated for use by access points in the 5 GHz band by IEEE Draft Supplement 802.11h, entitled “Spectrum and Transmit Power Management Extensions in the 5 GHz Band in Europe” (publication P802.11h/D2.1 of the IEEE Standards Department, Piscataway, N.J., July 2002), which is incorporated herein by reference. TPC in the 5 GHz band is required in some European countries in order to reduce interference with radar. It can also be used for interference reduction, range control and reduction of power consumption by access points and mobile stations.